1. Field of the Invention
The present invention relates to a method of generating transfer functions to be applied to the input data of an image to be printed by a printing apparatus using a group of colorants, each colorant being assigned to the rendering of a distinct color channel. The method includes the steps of determining a maximum total area coverage and measuring the tonal response of each colorant.
2. Description of Background Art
In many digital printing systems, dither patterns are used to create tone gradation. Some inkjet printing systems use three primary colorants in the form of cyan, magenta and yellow inks. These three colorants form a group of colorants. High quality printing systems may use the group of primary colorants, combined with a black colorant, or even more colorants. With dither patterns, it is possible to achieve on the print media a desired area coverage of a colorant for the purpose of rendering an input value of a given color channel. Typically, there is a non-linear relationship between the area coverage of a colorant deposited on the print media and the obtained optical density of the printed color. The input data of each color channel of an image to be printed must be corrected in order to obtain a desired output response of the printer. Most of the time, the correction is carried out such that the corrected input data appear to have a linear tone scale. Transfer functions are applied to input data of each color channel of an image to be printed to carry out the modification. A transfer function can be described by a polynomial function to be applied to the raw input data, or by using a look-up table, wherein modified input data are associated with raw input data.
In such printing systems, it is common practice to include colorant limitation methods or devices that make it possible to limit the amount of colorants deposited on the print media to a maximum total area coverage. Such limitation methods or devices are required to avoid excessive use of colorants. If excessive colorant is used, undesirable effects such as bleeding (an undesirable mixing of colorants along a boundary between printed areas of different colorants), cockling (warping or deformation of the print media), flaking and smearing may occur. This can lead to unacceptable print results, and even worse, may cause the print media to warp so much that it may damage mechanical parts of the printing apparatus. Therefore, a maximum total area coverage value is determined, depending on the colorants properties, print media properties, print applications printing conditions, etc.
A method for generating transfer functions of the above type is known from EP 1168822 A1. With the known method, once an operator has determined an overall upper ink limit (i.e. a maximum total area coverage) from a test pattern, individual ink limits (i.e. maximum individual area coverage values) are derived by dividing the total ink limit by three and apply the result to all colorants in the system. With the known method, the same maximum individual area coverage is thus attributed to each colorant. With the knowledge of the tonal scale curves and of the maximum individual area coverage value, the transfer functions can then be generated.
When using the known method, it appears that some printed colors are not saturated enough. A part of the gamut cannot be rendered by the printing apparatus in a satisfactory fashion, which can lead to very disappointing print results.